The present invention relates to the field of aircraft propelled by at least one jet engine fastened in the vicinity of their fuselage, in particular below a wing, and is aimed at a means for attenuating the noise emitted by the engine.
Noise emissions of civil turbofan engines have two main sources: the jet leaving the nozzle and the noise of the fan at the front. The present invention is concerned with the noise emitted toward the front of the aircraft by the engine.
This noise consists of the superposition of a line spectrum and of a wide-band spectrum: the line spectrum is composed of the multiple frequencies of the passage of the blades (FPB) on the one hand and of the multiple frequencies of the rotation (MFR) on the other hand; moreover, the wide-band spectrum is associated with the turbulent flows in the engine.
The FPB-type noise and the wide-band noise, above, essentially concern the regions on the ground around airports during takeoff and landing operations.
The MFR-type noise is associated with the shock cells generated in transonic flight at the leading edge of the fan blade and is present throughout the takeoff phase until cruising speed has been reached. This type of noise contributes both to the noise perceived on the ground and in the cabin. Specifically, in the case of a conventional air inlet, the MFRs radiate at low frequency and upstream of the engine with maximum directivity between the angles of 60° and 80° with respect to the axis of the engine. The noise is therefore little attenuated by the wall of the fuselage and is transmitted into the front part of the cabin.
The jet engine is housed in a nacelle comprising an air inlet duct oriented along the engine axis, with a generally annular leading edge at the front, having an aerodynamic profile in order to guide the air sucked in by the engine. In order to limit the effects of the noise emitted toward the front by the engine in the direction of the ground, it has been proposed to use nacelles having a beveled inlet profile. The engine inlet plane, also referred to as the hilite plane, is thus inclined toward the rear by elongating the lower part of the air inlet duct. The lower lip extends toward the front of the conventional inlet plane. This form allows a larger fraction of the noise emitted toward the front to be reflected and diverted upward. To achieve this aim, the shape of the inlet profile may also be a scoop shape. Whatever the case, the shape is determined in such a way as not to have a negative influence on the engine performance during the various flight phases. The noise cone envelope is thus modified. The half cone angle in the vertical plane of the lower part is smaller than the half cone angle in this same plane of the upper part. It follows that the region masked by the air inlet duct is more extensive in the lower part. Air inlets suitably configured to attenuate the noise on the ground during landing or takeoff phases are described, for example, in patents EP 823 547, EP 1308387, EP 1071608 and U.S. Pat. No. 3,946,830.
However, this manner of attenuating the noise does not make it possible to produce a favorable effect on the noise perceived in the cabin.